In semiconductor laser diodes, high power densities at the waveguide-to-mirror or waveguide-to-coating interface are usually considered responsible for a gradual degradation close to the facet. In addition, the maximum extractable intensity at the physical interface semiconductor-to-coating appears to be limited because of occurring breakdowns, so-called catastrophic optical damages (CODs) which are related to crystal defects in the semiconductor.
There have been many efforts to increase the maximum output power of semiconductor lasers.
Thompson in U.S. Pat. No. 3,943,462, “Antireflection coatings for injection lasers” and Gasser, Latta, Jakubowicz, Dietrich, and Roentgen in U.S. Pat. No. 5,940,424, “Semiconductor laser and method for making the same”, for example, describe laser diodes with multiple layers of dielectrics at the waveguide-to-mirror interface to reduce the power density at this interface.
For various reasons, antireflection mirrors or coatings with an optical thickness of λ/4, where λ is the emitting wavelength, have become important in semiconductor lasers. In the following, these coatings will be labelled quarter-wave coating, or QW coating.
Ueno et al. disclose in U.S. Pat. No. 6,285,700 how a phase-shifted λ/4 antireflection mirror, a phase-shifted QW coating, may help to increase the level at which the so-called catastrophic optical damage (COD) of semiconductor lasers occurs.
One important advantage of QW coatings is that their reflectivity is rather insensitive against thickness and wavelength deviations. This simplifies the manufacturing process insofar as the high uniformity and repeatability required for coatings with optical thickness other than λ/4, i.e. QW, is often difficult to achieve.
A special application of QW coatings are uncooled semiconductor lasers with an external cavity such as amplifiers. These lasers usually have antireflection coatings with a reflectivity close to zero (0.2% or less). Such a low reflectivity can only be achieved if the optical thickness of the coating is equal to λ/4. As a result of this, the diffraction index of the coating must be as close as possible to √{square root over (neff)}, where neff is the effective diffraction index of the light-emitting laser facet.
The reflectivity of a semiconductor laser facet is a function of the wavelength of the emitted light, the effective refractive index of the light-emitting facet area, and the thickness and the diffraction index of the coating or coatings deposited on the facet. In practice, the reflectivity is adjusted by controlling the properties of the coating since wavelength and refractive index of the emitting facet region are predetermined. Applying multiple layers of coatings leads to a highly undesirable complexity of the production process, therefore single layer coatings are preferred. For QW single layer coatings, the thickness is predetermined, therefore the reflectivity has to be adjusted by the refractive index of the coating material. The present invention shows how to adjust the refractive index for a material system so that QW coatings with a reflectivity between 0 and 10% are obtained in a reliable and easily controllable way.